This prospective investigation of families with Familial Hypocalciuric Hypercalcemia (FHH) will clarify the molecular genetics of the disorder and assess currently available diagnostic tests. Familial hypocalciuric hypercalcemia, which has also been known as familial benign hypercalcemia, is an autosomal dominantly inherited disorder. FHH begins with hypercalcemia in childhood and is usually asymptomatic throughout the patient's life. The major morbidity of the disorder is that it is often confused with primary hyperparathyroidism, resulting in unnecessary neck exploration.